The known methods of testing the payload of a satellite in orbit are most often based on the use of a test signal on an unmodulated carrier which is generated and transmitted on the uplink of the satellite in orbit by a ground station coupled to the test device. The satellite receives this test signal via a receiving antenna; the signal is propagated through a repeater and retransmitted to the ground station via a transmitting antenna. It is possible to characterize the response of the transmitting antenna of the satellite from measurements carried out on the signal transmitted on the downlink of the satellite.
There are many disadvantages of test methods based on the use of specific test signals.
The transmitting antenna test is limited to a part of the cover area of the receiving antenna. In fact, in order that the test method may be used, the ground station, which operates both the transmission of the test signal on the uplink and the acquisition of the signal retransmitted by the satellite on the downlink, must be positioned within the area of intersection of the cover areas of the receiving antenna and of the transmitting antenna of the satellite. Thus, it is not possible to test the transmitting antenna over the whole of its angular cover.
Moreover, the extraction of the contribution of the transmitting antenna alone necessitates the use of an automatic gain loop in order to compensate for the gain variations of the receiving antenna. This loop provides a constant level at the input interface of the transmitting antenna, which makes it possible to measure, on the ground, solely the variations of the transmitting antenna. However, as the radiation pattern of the receiving antenna is generally more directive than that of the transmitting antenna, the result of this is that the dynamic measuring range of the transmitting antenna under test is limited to the dynamic range available in the automatic gain control loop.
Moreover, the isolation measurements, that is to say the characterizing of the transmitting antenna for cover areas in which the gain of the antenna is very low, are also limited by the weak reception level on the receiving antenna.
Another problem associated with the use of test signals is that when it is desired to test the operation of a multi-beam transmitting antenna or of an antenna transmitting at several frequencies, this necessitates the generation of multi-carrier test signals and therefore a device for generating such signals, which increases the complexity of the test system.
Finally, the generation of a test carrier can interfere with other adjacent satellites and this necessitates overall frequency coordination.